# Pauli Matrix question!

1. Nov 29, 2012

### physics2000

1. The problem statement, all variables and given/known data

Pauli Spin matrices (math methods in physics question)

Show that D can be expressed as:

$$D=d_1\sigma_1+d_2\sigma_2+d_3\sigma_3$$

and write the $$d_i$$ in terms of D's elements, let D also be Unitary

2. Relevant equations

- Any 2x2 complex matrix can be written as :

$$M=m_1\sigma_1+m_2\sigma_2+m_3\sigma_3+m_0I$$ where "I" is the identity matrix

- Pauli spin matrix properties

-require that D have 0 trace

3. The attempt at a solution

no idea where to begin honestly. please dont ding me ! This is the first day i've ever dealt with pauli spin matrices

Last edited: Nov 29, 2012
2. Nov 29, 2012

### Dickfore

If it is simply given that D is a unitary matrix, then this is not true. For example, take D to be a unit matrix (which is clearly unitary). Then, as you correctly point out, it is not traceless, and cannot be represented in that way.

What is D supposed to be?

If D is unitary, then it may be written in an exponential form:
$$D = \exp(i X)$$
What property does X have if D is unitary? What if D has a unit determinant?

3. Nov 29, 2012

### physics2000

thats exactly what I'm confused about...assume D is traceless and not unitary, does this make more sense to you? I get what you are saying, and I completely agree

I assume from the question that D is supposed to be like M in the relevant equations section, in which the $$\sigma_i$$ correspond to the pauli matrices 1,2 and 3

4. Nov 29, 2012

### George Jones

Staff Emeritus
Even though not every unitary 2x2 matrix is traceless, there are many unitary matrices that are tracekess, i.e., there are many unitary matrices that can be written in the form D of the original post. For example, each Pauli matrix is unitary and traceless. So is $i \left( \sigma_1 + \sigma_3) \right)/\sqrt{2}$. So is ...

5. Nov 29, 2012

### physics2000

Thanks for the reply, I have no idea though how to answer the original question or start it. May I have some guidance :D?